This invention relates to processes and apparatus for the purification of wastewater in several sequential phases wherein the wastewater is aerated in a first phase and is then clarified in a second phase, which includes a sludge-return followed by a further aeration phase and a further sedimentation phase.
Such a process and apparatus are disclosed in German Offenlegungsschrift No. 28 57 345. The apparatus disclosed therein includes an activated sludge basin with a sludge-deposition zone and a post-aeration basin, as well as a deposition basin and an accumulation basin. The various basins have an essentially rectangular shape and are connected to each other with appropriate pipelines. The disadvantages of such arrangements are that they occupy a relatively large area, are expensive to build, and require expensive pipeline systems, fixtures and dams for moving wastewater from one basin to the next. Furthermore, the various basin sizes for the individual phases are fixed by the construction and cannot be adjusted after construction for different clarification processes. In addition, the construction requires excessive height differentials since there are head losses from basin to basin.